Extruders have been used conventionally for many years to process all types of polymeric materials and especially polyolefins. Generally, the polymer is melted and worked to some extent in the extruder and conveyed to a particular molding means so that it is in the proper state to be handled by that specific means.
In recent years, it has become known that various chemical reactions and modifications can take place when a polymer passes within the extruder. These modifications, in a manner, are hitching a ride or "piggy-backing" on a polymer which is already being routinely processed. Thus it is a tag-along process. The modifications can be accomplished in such a way that significant changes in the polymer composition and/or rheology can be effected.
There are numerous patents on such techniques of modifying polymers and many of these are specifically directed to the grafting of monomers to polymers that are being still processed. This background disclosure is restricted to those which are believed most relevant.
Very basic is British Pat. No. 679,562 which shows graft polymerization to polymers taking the place when the polymer is subjected to suitable mechanical working such as high-speed stirring, shaking, milling, kneading, grinding, ultrasonic vibrations or passage through filters or capillary tubes at high linear velocities, all of which lead to degradation of the polymer and create reactive sites.
U.S. Pat. No. 3,177,269 and U.S. Pat. No. 3,177,270 are pertinent patents which disclose the formation of graft copolymers by adding initiator and monomer to a polymer as it is being extruded. The polymer is "malaxed" to such a low degree that no degradation takes place.
U.S. Pat. No. 3,013,003 also discloses polymer degradation in an extruder and utilizes a stabilizer to prevent undue degradation. U.S. Pat. No. 3,270,090 is a variant of U.S. Pat. No. 3,177,269 in which the polymer is pre-irradiated before being subjected to extrusion.
U.S. Pat. No. 3,563,972 and U.S. Pat. No. 3,551,943 are relevant to polymer modification and extrusion using an oxygen-containing gas as an initiator to cause polymer breakdown at relatively high temperatures and without any special mixing modification. British Pat. No. 1,217,231 relates to grafting modifications wherein the amount of grafted polymer and homopolymer are controlled in some predefined ratio. British Pat. No. 1,042,178 teaches the preparation of modified polyolefins by shearing at extremely high shear rates in screw extruders, high-speed mills, roll mills and the like at a shear rate of at least 1500 reciprocal seconds.
Belgium Pat. Nos. 742,340, 742,338, 742,272 and 742,339 also relate to grafting processes of interest.
As will be seen hereinafter, none of these disclose, hint or suggest in any manner whatsoever Applicants' novel, unique and unobvious process and apparatus for modifying in a most unusual and desirable manner a polymer feedstock to in many instances result in novel polymers with very useful properties.